Transistors, such as MOSFETs, are often used as switches for switching power converters such as buck converters and boost converters. When used in power conversion applications, these switches are typically referred to as power switches.
FIG. 1 is a schematic of a transistor 100 that may be used as a power switch, and a controller 102 that is coupled to a gate of the of the transistor 100. The transistor 100 is operable in an on state, in which current is permitted to flow between its drain and source terminals, and an off state in which no current is permitted to flow between its drain and source terminals. In the on state, the transistor 100 has an associated on resistance Rdson, and as such, the transistor 100 in the on state may be represented by a resistor 104 as shown in FIG. 1.
For a lower on resistance Rdson of the transistor 100, there is a lower voltage drop across the drain and source terminals of the transistor 100 for a given current, and a lower amount of heat generated for a given current. A smaller voltage drop and less heat generation is beneficial in increasing the efficiency of a system implementing the transistor 100.
The on resistance Rdson of the transistor 100 is unsuitable for use in sensing current as the on resistance Rdson is susceptible to variation, for example due to variations in voltage, temperature and process/manufacturing.
FIG. 2A is a schematic of the transistor 100 coupled in series with a sense resistor 200. Common features between Figures share common reference numerals. The sense resistor 200, which has a resistance R, is used to measure a current. When an unknown current is passed through the sense resistor 200, there is a measurable voltage drop ΔV across the sense resistor 200. Using Ohm's Law, and the combination of the known resistance R and the voltage drop ΔV it is possible to calculate the instantaneous current flowing through the sense resistor 200 and the transistor 100.
As the transistor 100 is coupled in series to the sense resistor 200, the current flowing through the sense resistor 200 is equal to the current flowing through the transistor 100. Therefore, measurement of the current flowing through the sense resistor 200 is a suitable method to measure the current flowing through the transistor 100.
The sense resistor 200 may be referred to as a precision resistor, as the value of the resistance R is known to a sufficiently high degree of accuracy to determine the current, and the resistance R also does not vary substantially during normal operating conditions.
FIG. 2B is a schematic of the transistor 100 coupled in series with the sense resistor 200 and comprising an operational amplifier 202 and an analog to digital converter (ADC) 204. Common features between Figures share common reference numerals. The operational amplifier 202 and ADC 204 are used to measure the voltage drop ΔV across the sense resistor 200 and to convert the measured voltage drop ΔV from an analog to a digital value.
The sense resistor 200 may be implemented in a circuit to measure the current at a particular point in time, which is commonly referred to as the instantaneous current. Measurement of the instantaneous current is a necessary feature in many systems, however, addition of the sense resistor 200 lowers the efficiency of the system and produces extra heat.
The voltage drop ΔV across the sense resistor 200 is proportional to the current in the circuit and is also proportional to the resistance R of the sense resistor 200, in accordance with Ohm's Law. Reducing the resistance R of the sense resistor 200, for example by using a smaller sense resistor 200, will improve the efficiency of the circuit, as power loss and excess heat is reduced, but will also result in a proportionally smaller voltage drop ΔV. As the voltage drop ΔV is measured to determine the current, a smaller voltage drop ΔV means that more sophisticated voltage detection circuitry is required to measure the voltage drop ΔV to a sufficiently high accuracy that enables accurate determination of the current. This can increase the cost of implementing current sensing using a sense resistor 200.